On a cold bright Monday in March, my friend Vicki and I took a train journey together for the very last time. During our 20-year friendship, we had travelled together a lot but, this time, when we took the train from St Pancras to Zurich, I knew I would return alone. We were on our way to the Dignitas clinic, where Vicki would die less than a week later.

Vicki was fiercely independent and saw her life as an adventure. In her younger days, she cycled across Nepal, was shipwrecked off New Zealand and extradited from America. She was also a self-taught sailor, potter and toy maker. But 10 years ago she was diagnosed with COPD, an incurable type of emphysema. She was prescribed all sorts of medication and explored alternative therapies, but the breathlessness grew worse. Her energy levels fell to almost zero.

In 2004, she joined Dignitas and asked if I would go with her when the time was right. I agreed; part of me hoped that when the time was right, she might die gently in her sleep, but that was not to be.

Over the last few years, life became a struggle, each day a battle to get through. Vicki's world had become much narrower and she couldn't bear the thought of becoming a burden to friends. She contacted Dignitas, who reviewed her case and accepted her in July 2007. It was a slow process; people get the impression you fly out on Friday and it's over by the evening, but it doesn't work like that.

Last December, Vicki contacted Dignitas again to activate the "green light". She was hoping to have her escape route mapped out by January, but there was more paperwork, more documents. Almost three months later, we set off for the last time together to Switzerland. On Tuesday evening, Vicki had an appointment with a doctor who reviewed her case. She mustered all her reserves to show the doctor what she had once been – a feisty, independent woman. The doctor asked questions, took notes and set up another meeting for Thursday evening. We returned to our hotel, she needed hugs and we both needed a G&T. The next day was free, and we walked the quarter of a mile or less to the lake and took a boat trip. In the evening, she made the first round of farewell phone calls. After that it was more hugs, more G&Ts and listening to snippets of her life that she wanted to share.

Thursday would be Vicki's last whole day. We strolled by the river, talked and rested before the final doctor's appointment. He agreed to prescribe the drugs that would end her life on Friday.

That night I couldn't sleep. I knew I had to be strong, to put my sense of loss to one side and my feelings elsewhere. It was so odd being with someone and knowing they were going to die the next morning. But I felt I had to support my friend on the last journey that she insisted on taking, her final act of defiance.

In the morning, we took a taxi towards the outskirts of the city, where the Dignitas house is based, and two volunteers took us inside. There was more paperwork and then coffee in the garden.

Vicki had brought some Beethoven, which I put on the CD player. After some moments she asked to take the first drink, to relax her stomach. We hugged, and she took off her necklace and put it around my neck. Our host came in and offered her the drink, which she took. The music washed over us. Next came the last drink, the lethal one; a small glass was placed beside her, covered by a tissue.

Vicki was asked where she would like to sit and she chose the bed. She took the glass and drank steadily, without a tremor. She looked at me and said, "Oh, yuck" and from nowhere I replied, "Well, they won't do as last words." We both roared with laughter, our heads together. She rinsed her mouth with clean water and lay back on the pillows as I held her hand. We didn't know what to expect, and then she said, "I think something is happening; I'm off on the adventure." Her body relaxed and her eyes closed. I held her hand until she was safe from suffering, free.

When my friend made her choice and was accepted by Dignitas, she was empowered to live on – doing her best, enjoying her life and relishing the company of her friends for a further two and a half years. She lived as long as she possibly could and died bravely, just as she wished.